For driving conveyors it is common to employ shaft mounted gear units of the kind comprising a hollow shaft. To connect the gear unit in an aligned relationship with a foot-mounted motor and other components such as brakes it is common to employ a structural support bed as shown in FIG. 1 and comprising an integral assembly of welded components, or a casting, that provides base plate surfaces to which the gear unit and electric motor are respectively mounted. Standardisation of the design of the structural support beds is difficult because of the variation of sizes and heights of the motors and gear units which it may be wished to interconnect, and also the required spacing of the gear unit and motor, for example to accommodate different types of couplings and brakes. Furthermore, the manufacture, handling and transportation of the support beds is difficult, particularly for those structural support beds required to be of great length. Thus the present design is expensive and inconvenient.